1. Field of the Invention
The present invention relates in general to a high current electrical switch and a method of making it. It more particularly relates to such a high current electrical switch and method that has little or no need for a heat sink for many applications.
2. Background Art
The information contained in this section relates to the background of the art of the present invention without any admission as to whether or not such background art legally constitutes prior art.
Generally, the applications requiring direct current in the range of fifty to five hundred amperes will be found in systems operating in the voltage range of ten to a hundred volts. This window of electric power is found on board cars, trucks, boats, airplanes, recreational vehicles, telecom systems, photovoltaic systems wind power systems, oceanographic buoys, etc.
In recent years, MOSFET switches have been popular in computer systems, combining redundant power systems and/or batteries with a computing system such as a server, memory, and/or network. The limiting factor in most applications has been the resistance in the device from the drain to the source when the device is on, or the Rds(on) factor. This factor has been steadily reduced over the recent years by MOSFET manufacturers from sub-ohms to milli-ohms. Currently, devices with voltage ratings of 30 to 50 volts are available with an Rds(on) as low as 1.5 to 20 milli-ohms. As this parameter has asymptotically approached the one milli-ohm level, the package resistance (leadframe, wire bonds, etc.) limits much greater improvement. In the application of these high conductivity devices, the external resistances begin to swamp the improvement achieved in the MOSFET. The Rds(on) increases significantly (about a percent per degree Celsius) as the device temperature rises while conducting high currents.
To dissipate this temperature increase, the MOSFETs are placed on large costly heat sinks to carry away the heat produced as a result of the voltage drop. If these heat sinks are inadequate to avoid excessive heating of the MOSFETs, the MOSFETs may be destroyed. The common practice is to decrease the resistance between the MOSFET and its source and load as much as possible by increasing pad sizes, trace widths, trace thickness, and wire gauge, while decreasing trace lengths, wire lengths, and connector resistance within the limits imposed by the heat sink system.